Message 224033 - Python tracker

Here is a patch against 2.7 which get rid of reference loops in Tk variables and Font. This will fix not only ConfigDialog, but any similar user code.

In 3.4+ such reference loops are successfully resolved, but I think we should foreport this path to 3.4+ because it also fixes other minor bug: callbacks registered to trace variable now live while the variable lives, not while widget lives.